The optimistic view that antibiotics would enable permanent triumph over bacterial pathogens has been challenged by the emergence of antibiotic-resistant microorganisms. Kohanski et al. add a new twist to our understanding of bacterial resistance by showing that sublethal treatment with antibiotics can foster the development of antibiotic-resistant strains not only through selection of naturally occurring resistant variants but also by enhancing the rate of mutagenesis. Noting that some antibiotics stimulate bacterial production of reactive oxygen species (ROS) and that ROS damage DNA, Kohanski et al. treated Escherichia coli strain MG1655 with low concentrations of antibiotics and assessed mutation rate by the subsequent growth of colonies resistant to the antibiotic rifampicin. Whereas treatment with 1 μg/ml kanamycin (an aminoglycoside antibiotic) produced only a moderate increase in mutation rate, 1 μg/ml ampicillin (a β-lactam) or 50 ng/ml norfloxacin (a quinolone) elicited mutation rates comparable to those induced by 1 mM hydrogen peroxide. Analyses with a fluorescent indicator dye revealed that the change in mutation rate correlated with ROS production; moreover, the hydroxyl radical scavenger thiourea substantially reduced mutation induced by treatment with hydrogen peroxide or antibiotics. Furthermore, antibiotics had little effect on the mutation rate in bacteria grown under anaerobic conditions. Five days of culture in the presence of 1 μg/ml ampicillin increased the minimum inhibitory concentrations for ampicillin, norfloxacin, kanamycin, tetracycline, and chloramphenicol, an effect that was blunted with growth under anaerobic conditions. Intriguingly, sequential selection on plates containing different antibiotics after ampicillin treatment revealed that some ampicillin-treated bacteria developed resistance to other antibiotics (norfloxacin or kanamycin) without developing resistance to ampicillin. The authors thus conclude that one mechanism through which sublethal exposure to an antibiotic can lead to the emergence of resistant strains involves the ROS-dependent increase in mutation rate and that sublethal exposure to one antibiotic can independently affect resistance to others.